Cause these can get access to things Elon wants access to and will do what Elon wants. Musk does not want audit, he wants someone who will find exactly what Musk wants him to find and wont worry about legality or rules.

Musk is not concerned with producing false accusation for example. Obviously he could find corrupt auditors and probably did, but those are slower. They take more time to produce what was asked from them.